MTA-STS, or Mail Transfer Agent Strict Transport Security, is a crucial security standard designed to enforce TLS encryption for email in transit. It helps prevent downgrade attacks and Man-in-the-Middle (MITM) attacks, ensuring that emails between compliant servers are always sent over a secure connection. Essentially, it tells sending mail servers that your domain expects secure communication, and if it can't establish that, it should fail rather than send unencrypted. Without it, opportunistic TLS might downgrade to unencrypted if there are connection issues, leaving your emails vulnerable.
A common question that arises when configuring MTA-STS is whether wildcard MX entries are permitted or advisable. The MX field within your MTA-STS policy explicitly lists the mail servers authorized to receive mail for your domain. Getting this configuration right is paramount to avoiding mail delivery issues and maintaining the security benefits that MTA-STS offers. Let's delve into the specifics of wildcard MX entries and their implications for your email infrastructure.
The purpose of the MTA-STS mx field
Understanding the MTA-STS policy structure
The core of MTA-STS lies in its policy file, which is hosted on a web server and discovered via a DNS TXT record. This file specifies important parameters, including the protocol version, the mode of enforcement (e.g., enforce, testing, or none), the maximum age of the policy, and crucially, the mail servers (MX records) expected to handle inbound mail. The purpose of the MTA-STS 'mx' rule is to provide a whitelist of MX hosts that sending MTAs should trust as valid destinations for your domain's email.
When a sending server wants to deliver an email to your domain, it first checks for an MTA-STS policy. If found, it will attempt to connect to one of the MX hosts listed in that policy using TLS. If the connection fails to establish a secure TLS session or if the certificate presented by the receiving server is invalid or doesn't match the hostnames in the policy, the sending server will act according to the policy's mode. In 'enforce' mode, this typically means not delivering the email over an insecure connection, protecting against potential eavesdropping or tampering. You can read more about what happens if an MTA-STS policy is not found for further insights.
The explicit listing of MX entries is fundamental to the security model of MTA-STS. Each MX record specifies a fully qualified domain name (FQDN) of a mail server. This precision ensures that only your authorized, TLS-secured mail servers are accepted by sending MTAs. This also means understanding how to detect and verify MTA-STS policy changes is essential for maintaining a strong security posture. The MX field in an MTA-STS policy plays a critical role in this validation process.
The wildcard MX conundrum
The wildcard MX conundrum
The question of wildcard MX entries in MTA-STS policies brings up conflicting information from different providers and RFC interpretations. Some documentation, such as the RFC 8461 itself, suggests that a wildcard (e.g., *.example.com) might be allowed in the mx field. However, other authoritative sources and best practice recommendations strongly advise against their use. This divergence highlights a potential area of confusion and risk.
For instance, Microsoft's documentation for Exchange Online explicitly states that wildcards or '*' must not be used as the MX prefix in all MTA-STS scenarios, particularly for their services. Similarly, security.gov.uk advises against using wildcard MX records such as *.example.gov.uk, stating that this negates what MTA-STS is designed to achieve. The primary goal of MTA-STS is to enforce a highly specific and secure mail flow, which wildcards can undermine.
Why wildcards are problematic
Reduced security: A wildcard MX entry (*.example.com) means any subdomain could be considered a valid mail server. This broad scope weakens the explicit trust chain MTA-STS aims to build.
Increased attack surface: An attacker could potentially set up a fraudulent mail server under a subdomain that matches your wildcard MX record, then intercept emails.
Validation issues: The enforcement of TLS certificate validation becomes less precise. While a wildcard SSL certificate is acceptable for MTA-STS, a wildcard MX broadens the attack surface for server validation.
Recommended approach
Explicit MX entries: Always list the fully qualified domain names of your mail servers directly in your MTA-STS policy. This provides the highest level of security and clarity.
Match DNS MX records: The MX values in your MTA-STS policy should reflect TLS capable services that are also present in your public DNS MX records. This ensures consistency and proper validation.
MTA-STS validation: Ensure your MTA-STS validates MX records correctly to prevent misconfiguration from affecting deliverability.
While some technical implementations might tolerate wildcard MX entries, the consensus among security experts and major email providers leans heavily towards explicit MX listings. Using wildcards can introduce ambiguity and potentially undermine the very security assurances MTA-STS is designed to provide. Therefore, for robust email security and reliable deliverability, it's best practice to specify each MX record precisely.
Security implications and recommendations
Security implications and recommendations
The primary security concern with wildcard MX entries in an MTA-STS policy is that they can inadvertently broaden the scope of trusted mail servers. If an attacker manages to compromise a subdomain of your domain, and you have a wildcard MX record, they could potentially set up their own mail server under that subdomain. A sending MTA following your wildcard MTA-STS policy might then mistakenly deliver emails to this malicious server, believing it to be a legitimate destination. This scenario defeats the purpose of MTA-STS protecting against downgrade attacks, as it could lead to email interception.
Don't use wildcard MX entries!
To maintain the strongest possible security and avoid potential vulnerabilities, it is strongly recommended to avoid using wildcard MX entries in your MTA-STS policy. Always list each of your MX hosts explicitly. This practice aligns with the principle of least privilege, ensuring only known and verified mail servers can receive email for your domain under MTA-STS protection. This is also important because MTA-STS does not provide sender authentication, making receiver-side authentication even more critical.
When configuring your MTA-STS policy file, list the FQDNs of your MX records. If you use a third-party email service, these will typically be provided by your vendor. For example, if your MX records point to mail.example.com and backup.example.com, these exact hostnames should appear in your MTA-STS policy's mx directives.
Ensure that the TLS certificates used by your mail servers cover these specific hostnames. A wildcard SSL certificate (e.g., for *.example.com) is perfectly acceptable for securing your mail servers, provided it matches the explicitly listed MX hostnames. Understanding if MTA-STS works with any TLS certificate is a common point of confusion that should be clarified.
Verifying your MTA-STS policy
Verifying your MTA-STS policy
After implementing your MTA-STS policy, verification is a critical step. You need to ensure that the policy is discoverable, correctly formatted, and that your MX records are accurately reflected. Misconfigurations can lead to mail delivery failures or, worse, a false sense of security. Several tools are available online to check your MTA-STS setup, helping you confirm that everything is in order. Remember, the MTA-STS TXT record is how sending MTAs initially find your policy.
Suped offers comprehensive DMARC monitoring that also provides insights into your MTA-STS configuration. Our AI-powered recommendations help you to identify and fix issues, including any discrepancies or insecure configurations related to your MX entries. This holistic approach ensures your email infrastructure is not only compliant but also optimized for deliverability and security. The platform's real-time alerts ensure you're immediately aware of any changes or failures in your MTA-STS policy, preventing potential outages or security breaches. For example, Suped can help you check what DNS record type is used for MTA-STS policy discovery.
With Suped, you get a unified platform for DMARC, SPF, and DKIM monitoring, alongside blocklist and deliverability insights. This integration allows you to see how your MTA-STS policy interacts with your broader email authentication strategy. Our SPF Flattening feature helps overcome the 10-lookup limit, which can be crucial when your email ecosystem involves multiple vendors. For Managed Service Providers and agencies, our multi-tenancy dashboard makes managing MTA-STS and other email security policies across multiple client domains straightforward and efficient. Suped provides a generous free plan, making advanced email security accessible to everyone.
Conclusion
Embrace explicit configurations for enhanced security
While the technical specifications for MTA-STS may, in some interpretations, allow for wildcard MX entries, the overwhelming recommendation for secure and reliable email communication is to explicitly list all your mail servers in the MTA-STS policy. This precise approach minimizes the attack surface, strengthens the integrity of your email delivery, and aligns with the highest standards of email security best practices. By avoiding wildcards, you ensure that only your designated, TLS-secured mail servers are trusted by sending MTAs, thereby maximizing the protective benefits of MTA-STS. Regularly monitor your MTA-STS and other email authentication protocols to quickly address any issues.
